This invention relates to a method for controlling exposure in color photographic printers.
In order to print color photographs of a high quality, it is necessary to appropriately control the accurate measuring of a color original picture, and to precisely control the exposure and to appropriately determine the amount of light impinging onto a photosensitive material, and to optimally control the conditions in a photographic printer. Color original pictures may be color negative films, color reversal films, or color papers. Photosensitive materials may be color papers or color positive films.
In color negative paper system of an ordinary photographic printer currently used, a reference film negative (the film negative having an average density of the users' photographs) is usally used as a reference to control the printing conditions so as to secure printing at a predetermined density.
There are currently two exposure control processes in use; i.e. the additive color process and the subtractive color process. The additive color process is classified into the consecutive exposure process and the simultaneous exposure process of the primary colors. The additive process is the process of high-correction control. The subtractive process is adapted to consecutively control the exposure of RGB by restricting white light with filters (cut filters) of C (cyan), M (magenta) and Y (yellow). The subtractive process is classified into high-correction and lowered-correction types. The color compensation filter process is another process widely used in enlargers conventionally.
FIG. 1 shows an embodiment of the photographic printers of color compensation filter type wherein a film negative 1 is illuminated with light from a light source 3 via color compensation filters 2 of yellow (Y), magenta (M) and cyan (C). The light transmitted from the film negative 1 is guided onto a photographic paper 6 for printing via a lens unit 4 and a black shutter 5. The photographic paper 6 is reeled out from a supply reel 61. The photographic paper 6 after exposed at the printing section is processed at the processing section 7 for development, bleaching, fixing, washing and drying and then rolled on a take-up reel 62. At a location near the film negative 1 on the side of the lens unit 4 is arranged photosensors 8 such as photodiodes for detecting the image density for the three primary colors of red (R), green (G) and blue (B). The printing conditions are determined by the density detection signals for each of RGB from the photosensors 8 and the film negative 1 which has been conveyed to the printer section is printed under such conditions.
The filters 2 provided for color compensation may have a structure such as that shown in FIGS. 2A and 2B. Three filter plates 21 (21A through 21C) having a sectral quadrant shape are combined for each of the three colors of yellow (Y), magenta (M) and cyan (C). The light transmitted through a central light path 22 is controlled for each color by horizontal relative movement of each pair of filter plates 21A through 21C. The movement of the filter plates 21A through 21C are controlled by a control device (not shown) for respective colors. The filters plates 21A through 21C are approximated to the spectral transmittance distribution of the film negative dye so that exposure control can be performed precisely.
In such a color photographic printer, color failure or density failure is artificially corrected. It is emperically known that the LATD (Large Area Transmittance Density) balance in blue (B), green (G) and red (R) is substantially constant on a frame of standard color negative films. It is therefore a general practice in printing to measure the LATD of the three primary colors of BGR and control the exposure for the three primary color components at a constant value. In this way an excellent print of well-balanced colors can be obtained from a standard color film negative.
The above mentioned LATD control method, however, is not necessarily effective for the color film negative on which a specific color is dominant, and frequently produces defective prints with ill-balanced color. In order to deal with those problems, a photographic printer is generally equipped with color correction means of lowered-correction, normal-correction and high-correction levels to compensate the colors in negative films. More particularly, the lowered-correction method is a control process to apply correction in the amount of light of relatively low level against the relative changes in the LATD for three primary color components of a film negative and is suitable for the color failures caused by uneven color distribution of an object. The full-correction method is a control process to apply a certain amount of light to neutral (gray color) which is the result of the integration of three primary colors. This is suitable for correcting the negative films affected from different light sources or the negative films where latent images fade in the layer sensitive to a specific color.
The correction in the level of light suitable for the majority of negative films is referred to as a normal-correction which is lowered from the full-correction with respect to the amount of light to be exposed. The high-correction is at a level of light which is higher than the normal-correction level. Conventionally, the spectral characteristics of a light receiver of a printer, and the filter characteristics of the exposure control filter, etc. of a printer are insufficient to be used in full-correction, often resulting in the mixing of three primary color exposures and inevitably lowering the light level of correction. Full-correction can not be attained even if the intensity of the correction is increased so far as this condition prevails. Furthermore, high-correction is ineffective for color correction of different light sources under the conditions where full-correction can not be achieved. When color correction is changed with respect to the level of light, the print density is visually changed. Therefore, density as well as color should be corrected. This presents another difficulty.
Color compensation filters realize precise photomeasure and exposure control and therefore enable preparing conditions for full-correction. Then, a correction which is lowered from the full-correction with respect to its light amount enables performing an exposure control with good color balance on all of the films. Correction performance will also be improved for gradation changes in a film negative. This compensation filter type process can perform printer light source change correction at the same time as the exposure control. In the prior art, however, above mentioned advantages were not fully exploited because there was no process for precisely determining the exposure and controlling the filters.